Greenpeace also said its vessel was boarded in international waters and called on Russian President Vladimir Putin to release the crew immediately.

The ship is now expected to be taken to the Russian port of Murmansk.

Linda Harris, 55, from Devon, the mother of activist Alex Harris, 27, has not heard from her daughter since she got an email last Wednesday.

She said her daughter, who works in communications for Greenpeace, had a "passion for wildlife".

"She was very excited about this trip," she said. "I am just hoping... everything turns out for the best."

The Russian foreign ministry earlier accused the group of "aggressive and provocative" behaviour.

It said the actions of the activists who had tried to board Gazprom's Prirazlomnaya drilling rig on Wednesday "threatened people's lives and could lead to environmental catastrophe in the Arctic with unpredictable consequences".

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Media captionGreenpeace spokesperson Ben Stewart: "I got a call from a friend of mine last night, who was actually in the mess of the Arctic Sunrise, he said he was under armed guard"

Moscow also said that its coastguard vessel had to fire warning shots across the Dutch-flagged Arctic Sunrise.

The Dutch ambassador to Moscow was summoned to the foreign ministry over Greenpeace's action.

A foreign ministry spokesman in The Hague later told Dutch media the issue had "our full attention" and that contacts with the Russian authorities would be pursued over what had happened.

The Gazprom project is Russia's first effort to extract oil from the Barents Sea.

Prirazlomnaya is scheduled to begin production by the end of the year. Russia's economy and its recent growth depends to a large extent on income from its huge oil and gas deposits.